FIG. 3 shows a production method of a prior art box type quantum well structure. In FIG. 3, an ion cluster 6 including several tens of atoms, an ion cluster 7 including one to two hundred atoms and an ion cluster 8 including more than two hundred atoms are generated in a Ga ion cluster source 1. An acceleration electric field 3 is applied to the cluster ions by a grid for generating an acceleration electric field 2. Reference numeral 4 designates a GaAs substrate and an AlGaAs layer 5 is deposited thereon. Arsenic molecules 10 are supplied by an arsenic molecular beam source 9.
The operation and functions will be described hereinafter.
A plurality of clusters including several tens to several hundreds of Ga atoms are generated in the Ga cluster ion source 1, and each cluster is given monovalent positive charge equally and becomes a cluster ion. These ion clusters, that is, a cluster 6 including several tens of atoms, a cluster 7 including one to two hundred atoms and a cluster 8 including more than two hundred atoms emerge from the Ga cluster ion source 1, and thereafter an acceleration electric field 3 is applied thereto by the grid for an acceleration electric field 2. Then, the cluster ions are accelerated in the direction toward the GaAs substrate 4 on which the AlGaAs layer 5 is deposited and attach to the surface thereof. Meanwhile since the arsenic molecular beam source 9 continuously gives excessive arsenic molecules 10 to the substrate, constituent atoms of a Ga cluster which has attached onto the surface of the AlGaAs layer 5 are rearranged to be a GaAs box of a size in accordance with the number of Ga atoms, and as a result the GaAs box functions as a quantum well structure.
Since the prior art production method a box type quantum well structure is constituted as described above, several tens to several hundred Ga ion clusters are attached to the surface of the AlGaAs layer 5 at random. As a result, the GaAs box which functions as a quantum well is irregular in size and in quantum effect.
FIG. 4 shows a prior art cluster ion beam film forming device which has a function of selecting clusters in accordance with whether the number of constituent atoms is large or small, described in Japanese published Patent Application 62-27563. In FIG. 4, reference numeral 21 designates a bell jar. A cluster ion gun 23 which emits an ion beam 22, an accelerating electrode 24 for accelerating the ion beam 22 provided confronting to the cluster ion gun 23, a substrate holder 26 for holding a substrate 25, and a shutter 27 for cutting off the ion beam 22 are arranged in the bell jar 21. An electromagnet 28 for producing a magnetic field in the z direction of the figure is provided. A direct current power supply 29 for accelerating the ion beam 22 is connected with the accelerating electrode 24. Reference numeral 30 designates a quartz oscillator film thickness sensor.
The device will operate as follows.
The ion beam 22 emitted from the cluster ion gun 23 is accelerated toward the substrate 25 by the accelerating electric field which is generated by the accelerating electrode 24, and at the same time, the ion beam 22 is deflected by the magnetic field in the z direction which is produced by the electromagnet 28. Here, the extent that the ion beam 22 is deflected by the magnetic field increases as the number of constituent atoms of cluster becomes smaller. Accordingly, clusters including a large number of atoms attach to the substrate 25 without being deflected while the clusters including a small number of atoms are greatly deflected and do not attach to the substrate 25. In this way, this prior art device enables selection of the size of clusters which attach to the substrate. However, even when a box type quantum well structure is produced when the size of the cluster which attaches to the substrate is selected utilizing the above-described principle, only clusters having a small number of atoms are removed, and it is still impossible to obtain the same size of boxes so that the quantum effect does not vary. Furthermore, although the ionized clusters are selected in this method, the neutral particles which are not deflected by the magnetic field cannot be selected and clusters having a small number of atoms cannot be sufficiently selected.